Conventionally, the semiconductor light emitting element having a property of emitting the light having a emitting area from green to ultraviolet is researched. Such the semiconductor light emitting element is exemplified by a light emitting diode (LED) and a laser diode (LD). Such the light emitting diode and the laser diode are made of nitride semiconductor material of GaN type as follows.
AlxInyGazBuN
0≦x≦1
0≦y≦1
0≦z≦1
0≦u≦1
x+y+z+u=1
The semiconductor light emitting element such as blue light emitting diode and the ultraviolet light emitting diode are used in combination with the phosphor. The blue light emitting diode and the ultraviolet light emitting diode are configured to emit the blue light and the ultraviolet light, respectively. The blue light and the ultraviolet light are converted into the white light by the phosphor. In addition, the lighting device comprising the blue light emitting diode and the ultraviolet light emitting diode which is used in combination with the phosphor is also researched.
In a case where the semiconductor light emitting element which is made of the nitride semiconductor material of GaN type is manufactured, the substrate for crystalline growth is used. The substrate for crystalline growth is required to have a heat resistance property. In addition, the substrate for crystalline growth is required to have a coefficient of thermal expansion which is close to a coefficient of thermal expansion of the nitride semiconductor material. In light of this requirement, the sapphire wafer is used as the substrate for the crystalline growth. However, the sapphire wafer generally has a low electrical conductivity and a low heat conductivity. In addition to the above property, the sapphire wafer is hard. In addition, the sapphire wafer has low cleavage property. Therefore, the device having a semiconductor light emitting element comprising the sapphire wafer is limited its shape. In addition to the above limitation, the device having the semiconductor light emitting element comprising the sapphire substrate is required to be designed such that the device efficiently radiates the heat. That is to say, in order to emit a large amount of the light from one semiconductor light emitting element, there is a requirement to supply a large amount of the electrical current to the semiconductor light emitting element. When the semiconductor light emitting element receives a large amount of the electrical current, the semiconductor light emitting element generates a large amount of heat. Therefore, the device is required to have a design for effective heat radiation. In view of this requirement, conventionally, the sapphire wafer is polished to be thinned. In addition, conventionally, the sapphire wafer is removed from the nitride semiconductor. Specifically, first, the buffer layer is prepared on the upper surface of the sapphire wafer. The buffer layer is, for example, GaN which is grown under a condition of low temperature. Subsequently, the crystalline growth of the n-type nitride semiconductor layer is made on the upper surface of the buffer layer. Subsequently, the crystalline growth of the p-type nitride semiconductor layer is made on the upper surface of the n-type nitride semiconductor layer. Subsequently, the upper surface of the p-type nitride semiconductor layer is bonded with the supporting wafer. Subsequently, the laser beam such as the ultraviolet light is applied to the buffer layer through the sapphire wafer. Consequently, the sapphire wafer is separated from the n-type nitride semiconductor layer. Such the method of manufacturing the semiconductor light emitting element is researched.
However, the sapphire wafer has a coefficient of thermal expansion which is different from the coefficient of the thermal expansion of the nitride semiconductor layer. Therefore, in a case where the sapphire wafer is polished to be thinned, the nitride semiconductor layer receives the stress which is caused by the difference between the coefficient of the thermal expansion of the sapphire wafer and the coefficient of the thermal expansion of the nitride semiconductor layer. The stress causes the warpage of the sapphire wafer and the multilayered nitride semiconductor layer. The warpage causes the crack of the sapphire wafer and the multilayered nitride semiconductor layer.
In addition to the above, when the sapphire wafer is separated from the multilayered nitride semiconductor layer, the laser beam is applied to the multilayered nitride semiconductor layer through the sapphire wafer. That is, the buffer layer of the multilayered nitride semiconductor layer receives the laser light. When the buffer layer receives the laser light, GaN is dissolved into Ga and N. When GaN is dissolved, N2 is evolved. N2 gas is evolved between the multilayered nitride semiconductor and the sapphire wafer. N2 gas applies the gas pressure to the multilayered semiconductor layer. Therefore, the gas pressure of N2 gas causes the crack of micrometer order to the multilayered nitride semiconductor layer. The crack causes the leakage of the electrical current. Therefore, the semiconductor light emitting element which is manufactured by the above has low yield ratio.
Japanese patent application publication No. 3518455B (hereinafter referred as to Patent literature 1) and No. 3795765B (hereinafter referred as to Patent literature 2) disclose the technical solution for solving the above problem. The patent literature 1 and the patent literature 2 disclose the step of preparing the space, in advance, to the boundary of the transparent crystal wafer and the nitride semiconductor layer. The space absorbs the gas pressure which is caused by N2 which is evolved in a position between the transparent crystal wafer and the nitride semiconductor layer. Patent literature 1 and Patent literature 2 disclose the following steps. First, the foundation layer including the buffer layer which is a part of the multilayered nitride semiconductor layer is prepared by MOVPE method, whereby the buffer layer is made by the crystalline growth. The buffer layer is made of GaN. Subsequently, by the photolithograph and etching, patterning is made on the upper surface of the foundation layer and the sapphire wafer. Subsequently, the n-type nitride semiconductor layer and p-type nitride semiconductor layer is prepared on the upper surface of the buffer layer and the sapphire layer. The n-type nitride semiconductor layer and p-type nitride semiconductor layer are prepared by the crystalline growth of the epitaxial lateral overgrowth. Consequently, the space is formed in the boundary between the sapphire wafer and the multilayered nitride semiconductor layer. Subsequently, the laser beam is applied to the buffer layer through the sapphire wafer, whereby the sapphire wafer is removed. In addition, Japanese patent application publication No. 3525061 B (Patent literature 3) discloses the step of forming the irregularity to the upper surface of the photolithograph and the etching. In Patent literature 3, first, the irregularity is formed to the upper surface of the sapphire wafer. Subsequently, the nitride semiconductor layer is formed. Subsequently, the step of applying the laser beam to the nitride semiconductor layer through the sapphire wafer is performed. Consequently, the sapphire wafer is removed from the multilayered nitride semiconductor layer.
However, the method disclosed in Patent literature 1 and Patent literature 2 includes two steps of the crystalline growth. Therefore, in a case where the multilayered nitride semiconductor layer is manufactured according to the method disclosed in Patent literature 1 and Patent literature 2, there is a need to take a long manufacturing time. In addition to this need, this method required much cost. In addition, after the foundation layer is formed, the sapphire wafer is taken out from the chamber having a vacuum state. Subsequently, the first forming step of applying the treatment of the foundation layer. Subsequently, the second forming step of forming the semiconductor layer on the upper surface of the foundation layer in the chamber having the vacuum state, again. It should be noted that the sapphire wafer is taken out from the chamber having the vacuum state in the interval period between the first forming step and the second forming step. When the sapphire wafer is taken out to the outside, there is a possibility of that the impurity is adhered to the surface. As a result, there is a possibility of mixing the needless impurity with the multilayered nitride semiconductor layer. That is, when the sapphire wafer is taken out to the outside, there is a possibility of quality loss of the multilayered nitride semiconductor layer.
In addition, in Patent literature 3, the sapphire wafer is provided at its upper surface with an irregularity. In addition, the multilayered nitride semiconductor layer which includes the buffer layer is formed on the upper surface of the sapphire wafer. The multilayered nitride semiconductor layer is formed by the crystalline growth which is different from the epitaxial lateral overgrowth. That is, the irregularity of the sapphire wafer exerts the influence of the crystalline of the multilayered nitride semiconductor in the early step of the crystalline growth.
In view of the above problem, Japanese patent application publication No. 2007-299935A (hereinafter referred to as Patent literature 4) discloses another method of manufacturing the semiconductor light emitting element. Patent literature 4 discloses the step of forming a multilayered nitride semiconductor layer by the crystalline growth on the upper surface of the sapphire wafer. Subsequently, the groove forming step of forming the groove extending from the upper surface of the multilayered nitride semiconductor layer to the upper surface of the sapphire wafer is performed. Subsequently, the laser light is applied to the multilayered nitride semiconductor layer through the sapphire wafer. In this manner, the sapphire wafer is removed from the multilayered nitride semiconductor layer.
According to the manufacturing method disclosed in Patent literature 4, the crystalline of the multilayered nitride semiconductor layer is kept at high level. In addition, it is possible to prevent the generation of the crack of micrometer order due to N2 gas which is evolved when the sapphire wafer is removed.
However, when the semiconductor light emitting element is manufactured by the method disclosed in Patent literature 4, the minute irregularity is formed adjacent to the groove when the groove is formed. The minute irregularity decreases the bonding force between the multilayered nitride semiconductor layer and the supporting wafer. Therefore, when the semiconductor light emitting elements are produced by the cutting, there is a possibility of degradation of the reliability of the bonding between the multilayered nitride semiconductor layer and the supporting wafer which is a part of the supporting substrate.